A. Field of the Invention
The present invention relates to a method of plating on a base plate composed of a glass material, a method of manufacturing a disk substrate for a magnetic recording medium using the plating method, and a method of manufacturing a perpendicular magnetic recording medium using the method of manufacturing a disk substrate. In particular, the methods are beneficially applied to a magnetic recording medium mounted on a hard disk drive.
B. Description of the Related Art
In recent years, hard disk drives have been often used for a memory device in computers or digital household appliances. In the case of a longitudinal magnetic recording system, a magnetic disk (hard disk) as a magnetic recording medium mounted on the hard disk drive is generally manufactured by the following procedure. A Ni—P layer is formed on the surface of a nonmagnetic substrate with a disk shape by an electroless plating method. The surface of the Ni—P layer is subjected to necessary smoothing and texturing treatments. Then, an underlayer of a nonmagnetic metal, a magnetic layer of a ferromagnetic alloy thin film, a protective layer, and other layers are sequentially formed on this surface by a sputtering method or other techniques.
Traditionally, an aluminum alloy has been used as the material of the nonmagnetic substrate. Recently, hard disk drives are rapidly evolving to have higher capacity, smaller size, and lighter weight. In conjunction with this trend, a magnetic disk is required to have higher flatness, smaller diameter, and less thickness than previously. Conventional substrates of an aluminum alloy can hardly meet those requirements of the market. Thus, glass is being used for a substrate material.
A glass substrate must exhibit surface characteristics similar to those in an aluminum substrate by forming a Ni—P layer on the surface to obtain a magnetic disk exhibiting favorable performance. However, it is technically difficult to form a plating film with satisfactory adhesivity, homogeneity, and smoothness on a base plate composed of a glass material by an electroless plating method. To solve this problem, various methods have been proposed as pre- and post-treatments for the electroless plating.
In one example of such methods, electroless plating is conducted after a treatment using an aqueous solution containing palladium chloride and tin (II) chloride, and a treatment using an aqueous solution of alkali carbonate, an aqueous solution of alkali hydrogen carbonate, or a mixture of these aqueous solutions. (See Japanese Unexamined Patent Application Publication No. H1-176079.) In another method, electroless plating is conducted after a two-stage etching treatment using a chromic acid—sulfuric acid mixed solution and a nitric acid solution, an etching treatment using a strong alkaline solution, a sensitization treatment using dilute tin (II) chloride, and an activation treatment using a silver salt solution and a palladium salt solution. (See Japanese Unexamined Patent Application Publication No. S53-19932.) In another example, electroless plating is conducted after cleaning using a warm liquid of sulfuric acid and potassium dichromate, sensitization using tin (II) chloride acidified with hydrochloric acid, and activation using a palladium chloride solution. (See Japanese Unexamined Patent Application Publication No. S48-85614.) In still another method, electroless plating is conducted after alkali degreasing, etching using hydrofluoric acid, sensitization using a tin (II) chloride solution, and activation using a palladium chloride solution.
Japanese Unexamined Patent Application Publication No. H7-334841 proposes a method of electroless plating to form a Ni—P layer exhibiting sufficient adhesivity and smoothness on a glass substrate to obtain a favorable magnetic disk. In this method, electroless Ni—P plating is conducted after the pre-treatments of: sufficiently degreasing the glass substrate, etching to enhance anchoring effect, removing contamination that is produced in the etching process and adhered on the substrate surface, using a surface modulation process to chemically homogenize the substrate surface, using a sensitizing treatment, and using an activation treatment. The method preferably uses an aqueous solution containing hydrofluoric acid and potassium hydrofluoride for the etching solution, hydrochloric acid for removing the surface contaminant, and an aqueous solution containing sodium methoxide for the surface modulation.
Japanese Unexamined Patent Application Publication No. 2000-163743 proposes a method of forming an electroless Ni—P plating layer on a glass substrate for a magnetic disk. In this method, electroless Ni—P plating is conducted after sequential treatments on a glass substrate surface including: alkali degreasing treatment using a potassium hydroxide solution, etching treatment using hydrofluoric acid, treatment with warm pure water, silane coupling agent treatment, activator treatment using an aqueous solution of palladium chloride, and accelerator treatment using an aqueous solution of sodium hypophosphite. Heat treatment is conducted after the electroless Ni—P plating.
Meanwhile, a perpendicular magnetic recording system is drawing attention in place of a conventional longitudinal magnetic recording system as a technology to attain high density of magnetic recording. In particular, a double layer perpendicular magnetic recording medium as disclosed in Japanese Patent Publication No. S58-91 is known as a perpendicular magnetic recording medium for achieving high density recording. The double layer perpendicular magnetic recording medium is provided with a soft magnetic film called a soft magnetic backing layer under a magnetic recording layer that records information. The soft magnetic backing layer easily permeates the magnetic flux generated from the magnetic head and exhibits high saturation magnetic flux density Bs. The double layer perpendicular magnetic recording medium increases the intensity and gradient of the magnetic field generated by the magnetic head, improving recording resolution and increasing leakage flux from the medium.
A soft magnetic backing layer generally uses a film 200 nm to 500 nm thick that is formed by a sputtering method and is composed of a Ni—Fe alloy, an Fe—Si—Al alloy, or an amorphous alloy of mainly cobalt. However, forming such a relatively thick film by a sputtering method is inappropriate from the viewpoints of manufacturing costs and mass productivity. To solve this problem, use of a soft magnetic film formed by an electroless plating method has been proposed for a soft magnetic backing layer. Japanese Unexamined Patent Application Publication No. H7-66034, for example, proposes to produce a NiFeP film by a plating method on a disk substrate of an aluminum alloy provided with a nonmagnetic NiP plating film and to use for a soft magnetic backing layer.
Digest of 9th Joint MMM/Intermag Conference, EP-12, p. 259 (2004) proposes a CoNiFeP plating film formed on a glass substrate. Digest of 9th Joint MMM/Intermag Conference, GD-13, p. 368 (2004) proposes a soft magnetic NiP plating film formed on an aluminum alloy disk substrate provided with a nonmagnetic Ni—P plating film.
If a soft magnetic backing layer forms a magnetic domain structure and generates a magnetic transition region called a magnetic domain wall, the noise called spike noise that is generated from this magnetic domain wall is known to degrade the performance as a perpendicular magnetic recording medium. Consequently, formation of the magnetic domain wall must be suppressed in a soft magnetic backing layer.
The NiFeP plating film mentioned previously is liable to form a magnetic domain wall. Thus, Journal of Magnetic Society of Japan (in Japanese), Vol. 28, No. 3, p. 289-294 (2004) discloses that the domain wall formation needs to be suppressed by forming a Mnlr alloy thin film on the plating film by a sputtering method. Formation of a magnetic domain wall in the CoNiFeP plating film mentioned previously is disclosed to be suppressed by conducting plating in a magnetic field. A soft magnetic NiP plating film is said not to generate spike noise.
Japanese Unexamined Patent Application Publication No. H2-18710 proposes that the generation of spike noise be suppressed by forming a backing layer composed of cobalt or a cobalt alloy having coercivity Hc of 30 to 300 Oe so as to exhibit magnetic anisotropy along the circumferential direction of the disk substrate. While the backing layer in this method is formed by a dry process such as a sputtering method, an evaporation method, or the like, Japanese Unexamined Patent Application Publication No. H5-1384 proposes a method of forming a Co—B film that exhibits an Hc of at least 30 Oe and can suppress spike noise, by a plating method. The film is suggested to be possibly used for a soft magnetic backing layer.
The NiFeP plating film mentioned previously needs to suppress formation of a magnetic domain wall by forming a Mnlr alloy thin film on the plating film employing a sputtering method to suppress spike noise. The requirement for adding a new film by means of a sputtering method for suppressing a magnetic domain wall detracts from the merit of the plating method in production costs and mass productivity.
In the CoNiFeP plating film mentioned previously, application of a homogeneous magnetic field to a substrate in a plating bath is difficult in a practical manufacturing process. The mass productivity is also liable to be impaired. An iron-containing plating film, exhibiting high Bs value, is favorable for a soft magnetic backing layer. However, since iron forms an ion of ionic valence of two and an ion of ionic valence of three, securing the stability of a plating bath is known to be generally difficult. Thus, the iron-containing plating film is also inferior in mass productivity.
As to the correlation between coercivity and magnetic domain wall formation of the soft magnetic backing layer formed by a plating method, it has been clarified that a coercivity value of the plating film of not smaller than 30 Oe cannot completely prevent the magnetic domain wall formation, although some tendency of suppression was observed. It has been further clarified that the increase of the coercivity deteriorates the read/write performance.
As described above, for a disk substrate of a magnetic recording medium mounted on a hard disk drive, a glass disk substrate using crystallized glass or chemically strengthened glass is used as well as an aluminum alloy substrate provided with a nonmagnetic NiP plating film. The glass substrates, having high strength, are mainly used in a magnetic recording medium of a mobile hard disk drive, which needs high shock resistance. The above-described electroless plating method for forming a soft magnetic plating film as a backing layer is effective to improve the productivity also in a glass disk substrate for a perpendicular magnetic recording medium.
The electroless plating films composed of a nonmagnetic Ni—P alloy have been practically used in an aluminum alloy substrate for hard discs, and the manufacturing method for mass production and the surface smoothing technique by polishing are well known. Consequently, in a glass substrate, too, if a nonmagnetic or soft magnetic plating layer that has good adhesivity as an underlayer and satisfactory smoothness can be formed by means of an electroless plating method with a sufficient thickness (at least 1 μm) for obtaining a magnetic disk that performs well, the glass substrate with an electroless plating film is very promising for a substrate of a magnetic recording medium from the view point of production costs.
Unfortunately, the known methods of electroless plating as described above have failed to form, on a glass substrate, a soft magnetic plating film of Co—Ni—P, Ni—P, Ni—Fe—P, or Co—Ni—Fe—P, and a nonmagnetic plating film of Ni—P with a sufficient thickness (in the range of 1 μm to 3 μm) for obtaining a favorable magnetic disk and with satisfactory adhesivity, homogeneity, and smoothness at that thickness.
An underlayer of Ni—P or the like is known to be formed by a sputtering method, too. It is, however, difficult to form an underlayer directly on a glass substrate since adhesivity between glass and metal is poor. To cope with this difficulty, a layer containing titanium or chromium, which exhibit relatively good adhesivity with glass among the metals, needs to be formed on the glass substrate, and an underlayer is formed on the adhesion layer of titanium or chromium. The titanium or chromium of the adhesion layer in this method does not exhibit enough adhesivity. So, when the underlayer or adhesion layer is thick, the adhesivity deteriorates due to the stress caused by the difference of expansion coefficients. Perpendicular magnetic recording media, which are being actively developed recently, need a relatively thick layer of soft magnetic backing layer in the range of 0.2 μm to 3.0 μm. The soft magnetic backing layer, when deposited by sputtering, involves a problem of degradation of adhesivity and in addition a problem of high costs.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.